Emails suddenly not working -- centos3 and ispconfig

Besides adding new clients we have not changed any setting on our ispconfig server running on centos 3. We are getting reports that people are not receiving any email. They dont send through us so outgoing mail is fine.

Does anyone know of any centos updates that could have caused the postfix/dovecot services to stop responding. According to ispconfig the services are running fine. I will continue to investigate but any help appreciated.